The Corvette master cylinder from a 1977-1982 model is an AC Delco #18M91, has an 1-1/8" bore, and is designed for a disc/disc system. If you choose to use it in a disc/drum application, you'll want to add a 10# residual valve to the rear line to keep the shoes close to the drums. There are probably better MC choices for a disc/drum setup that will save you the expense of thet residual valve...however if you're going to convert the rear to discs in the future anyways, it may not be a bad choice.